Front Matter

There are two reported cases where an upper temperature limit for activity is reached

within the one-phase region and below the thermostable limit of the lipase. The first

is for glycerol-laurate esterification catalyzed byR. delemarlipase (Hayes and Gu-

lari, 1991). The second is forC. rugosacatalyzed esterification of oleic acid and

octanol (Ayyagari and John, 1995). In addition, enhanced thermostability has

been reported for lipases encapsulated in lecithin w/o-MEs (Chen and Chang, 1993).

Enzyme kinetics for esterification and hydrolysis reactions employing several

surfactant systems have been described by the Ping Pong Bi-Bi mechanism (Pra-

zeres et al., 1993b; Stamatis et al., 1993c; 1995; Yang and Gulari, 1994; Nagayama

et al., 1996; Jenta et al., 1997b), which is applicable for lipase reactions in several

media types (Malcata et al., 1992). This mechanism features sequential steps: the

formation of an acyl-enzyme intermediate, followed by the reversible attack by var-

ious nucleophiles (e.g., alcohols, polyol monoesters, water). The model, when ap-

plied to w/o-ME systems, frequently requires incorporation of competitive inhibition

term for FFA (Tsai and Chiang, 1991; Prazeres et al., 1993b; Yang and Gulari, 1994;

Stamatis et al., 1995; Jenta et al., 1997b) and alcohol (Stamatis et al., 1995), and a

noncompetitive inhibition term for surfactant (Marangoni et al., 1993). For most

cases, in agreement with kinetic models, the reaction rate is proportional to the over-

all enzyme concentration (Han et al., 1987a; Hayes and Gulari, 1990; Miyake et al.,

1993a; Crooks et al., 1995a; Rees and Robinson, 1995).

The optimal alkanol chain length for fatty acid-n-alkanol esterification for reac-

tions catalyzed in w/o-MEs can be quite different from that encountered in alternate

reaction media such as o/w emulsions (Hayes and Gulari, 1990). The differences

reflect the partitioning of the alkanol molecules in the respective reaction media.

Of great interest, the optimal chain length varies greatly with lipase type in a given

w/o-ME system. For instance, lipases from theRhizopusandPseudomonasfamilies

prefer 1-propanol (Hayes and Gulari, 1990; Stamatis et al., 1993a; 1993d; 1995),C.

rugosaprefers 1-butanol or 1-hexanol, depending on surfactant system (Morita et al.,

1994; Hayes and Gulari, 1990), andPenicillium simplicissimumprefers 1-nonane

(Stamatis et al., 1993d). It has been suggested that this difference may reflect the

position of the enzyme in the w/o-ME droplet. For instance, from light scattering

experiments (Hayes and Gulari, 1990; Hayes, 1991), it is speculated that 1-propanol

solubilizes in all three nanodomains (aqueous core, interfacial region, and bulk sol-

vent), 1-butanol solubilizes strongly in the interfacial region, and that the alkanols

partition more strongly to the bulk solvent and penetrate the surfactant layer less

strongly as the chain length increases further. According to this hypothesis,Rhizo-

pusandPseudomonaslipases solubilize more strongly in the interior water pools and

P. simplicissimumsolubilizes to a greater extent in the surfactant tail region. The

alkanol chain length-activity profile is reported to change withwo(Schlatmann

et al., 1991), surfactant type (Stamatis et al., 1995; Avramiotis et al., 1996) and

substrate type (Hayes and Gulari, 1990).

The composition of the w/o-ME system greatly impacts the enzyme kinetics. For

instance, a bell-shaped relationship between the parameterwo, the water-surfactant

molar ratio, and the observed enzymatic activity, commonly occurs for several en-

capsulated enzymes, including lipases. Numerous examples are provided in the tech-

nical literature. An example is given in Figure 3 forC. rugosalipase. The parameter

wofor many surfactant systems is proportional to the size of the w/o-MEs. The op-

3.2 Lipases encapsulated in water-in-oil microemulsions 51